Biodegradable materials are presently in high demand for applications in packaging materials. Commonly used polystyrene (“Styrofoam” (Trademark)), polypropylene, polyethylene, and other non-biodegradable plastic-containing packaging materials are considered detrimental to the environment. The use of such non-biodegradable materials will decrease as government restrictions discourage their use in packaging applications. Biodegradable materials that are flexible, pliable and non-brittle are needed in a variety of packaging applications, particularly for the manufacture of shaped biodegradable containers for food packaging.
For such applications, the biodegradable material must have mechanical properties that allow it to be formed into and hold the desired container shape, being resistant to collapse, tearing or breaking. In addition, biodegradable materials with resiliency, compressibility and bulk density comparable to, “Styrofoam” (Trademark) “peanuts” are needed in loose-fill packaging applications. Further, biodegradable sheet materials having properties comparable to polypropylene and polyethylene materials are needed, for example, in laminating packaging materials.
Starch is an abundant, inexpensive biodegradable polymer. A variety of biodegradable starch-based materials have been proposed for use in packaging applications. Conventional extrusion of starch produces expanded products that are brittle, sensitive to water and unsuitable for preparation of packaging materials. Attempts to prepare starch-based products with flexibility, pliability or resiliency and other mechanical properties acceptable for various biodegradable packaging applications have generally focused on chemical or physico-chemical modification of starch, the use of expensive high amylose starch or mixing starch with synthetic polymers to achieve the desired properties while retaining biodegradability. A number of references relate to extrusion, and injection molding of starch-containing compositions.
U.S. Pat. Nos. 4,133,784, 4,337,181, 4,454,268, 5,322,866, 5,362,778, and 5,384,170 relate to starch-based films that are made by extrusion of destructurized or gelatinized starch combined with synthetic polymeric materials. U.S. Pat. No. 5,322,866 specifically concerns a method of manufacture of biodegradable starch-containing blown films that includes a step of extrusion of a mixture of raw unprocessed starch, copolymers including polyvinyl alcohol, a nucleating agent and a plasticizer. The process is said to avoid the need for preprocessing of the starch. U.S. Pat. No. 5,409,973 reports biodegradable compositions made by extrusion from destructurized starch and an ethylene-vinyl acetate copolymer.
U.S. Pat. No. 5,087,650 relates to injection-molding of mixtures of graft polymers and starch to produce partially biodegradable products with acceptable elasticity and water stability. U.S. Pat. No. 5,258,430 relates to the production of biodegradable articles from destructurized starch and chemically-modified polymers, including chemically-modified polyvinyl alcohol. The articles made are said to have improved biodegradability, but to retain the mechanical properties of articles made from the polymer alone. U.S. Pat. No. 5,292,782 relates to extruded or molded biodegradable articles prepared from mixtures of starch, a thermoplastic polymer and certain plasticizers. U.S. Pat. No. 5,403,875 relates to blends of starch with acrylic polymers which are melt-processed into thermoplastic articles. U.S. Pat. No. 5,393,804 relates to biodegradable compositions made from starch melts containing converted starch and plasticizers including polyvinyl alcohol. U.S. Pat. No. 5,095,054 concerns methods of manufacturing shaped articles from a mixture of destructurized starch and a polymer.
U.S. Pat. No. 4,125,495 relates to a process for manufacture of meat trays from biodegradable starch compositions. Starch granules are chemically modified, for example with a silicone reagent, blended with polymer or copolymer and shaped to form a biodegradable shallow tray.
U.S. Pat. No. 4,673,438 relates to extrusion and injection molding of starch for the manufacture of capsules. U.S. Pat. No. 5,427,614 also relates to a method of injection molding in which a non-modified starch is combined with a lubricant, texturizing agent and a melt-flow accelerator.
U.S. Pat. Nos. 4,863,655, 5,035,930 and 5,043,196 report expanded starch materials in which the starch has at least 45% by weight amylose (high amylose materials). Expanded high amylose starch compositions have also been ‘modified by reaction with oxiranes (e.g. ethylene oxide) and include the commercially available “ECO-FOAM” (Trademark) in which the starch is modified by reaction with propylene oxide. Expanded packaging materials made from high amylose starch are too expensive for practical commercial use. U.S. Pat. No. 5,314,754 of Knight (May 24, 1994) reports the production of shaped articles from high amylose starch.
EP published application EP 712883 (published May 22, 1996) relates to biodegradable, structured shaped products with good flexibility made by extruding starch having defined large particle size (e.g., 400 to 1500 microns). The application, however, only exemplifies the use of high amylose starch and chemically-modified high amylose starch.
U.S. Pat. No. 5,512,090 refers to an extrusion process for manufacture of resilient, low density biodegradable packaging materials, including loose-fill materials, by extrusion of starch mixtures comprising polyvinyl alcohol (PVA) and other ingredients. The patent refers to a minimum amount of about 5% by weight of PVA. U.S. Pat. No. 5,186,990 reports a lightweight biodegradable packaging material produced by extrusion of corn grit mixed with a binding agent (guar gum) and water. Corn grit is said to contain among other components starch (76–80%), water (12.5–14%), protein (6.5–8%) and fat (0.5–1%). U.S. Pat. No. 5,208,267 reports biodegradable, compressible and resilient starch-based packaging fillers with high volumes and low weights. The products are formed by extrusion of a blend of non-modified starch with polyalkylene glycol or certain derivatives thereof and a bubble-nucleating agent, such as silicon dioxide. U.S. Pat. No. 5,252,271 of Hyrum (Oct. 12, 1993) reports a biodegradable closed cell light weight packaging material formed by extrusion of a modified starch. Non-modified starch is reacted in an extruder with certain mild acids in the presence of water and a carbonate compound to generate CO2. Resiliency of the product is said to be 60% to 85%, with density less than 0.032 g/cm3.
U.S. Pat. No. 3,137,592 relates to gelatinized starch products useful for coating applications produced by intense mechanical working of starch/plasticizer mixtures in an extruder. Related coating mixtures are reported in U.S. Pat. No. 5,032,337 to be manufactured by extrusion of mixture of starch and polyvinyl alcohol. Application of thermomechanical treatment in an extruder modifies the solubility properties of the resultant mixture which can then used as a binding agent for coating paper.
While significant progress has been made toward non-brittle, starch-based biodegradable packaging materials, there nevertheless remains a significant need for lowering production costs and improving the physical and mechanical properties of extruded starch materials.